/Engine/Plugins/Runtime/GameplayAbilities/Source/GameplayAbilities/Public/GameplayEffectTypes.h
Sets the instigator and effect causer. Instigator is who owns the ability that spawned this, EffectCauser is the actor that is the physical source of the effect, such as a weapon. They can be the same.
virtual void AddInstigator ( class [AActor](API\Runtime\Engine\GameFramework\AActor) * InInstigator, class [AActor](API\Runtime\Engine\GameFramework\AActor) * InEffectCauser )